1. Field of Invention
This invention relates to a method for the removal of contaminates from granular solids and, in particular, to the regeneration of activated carbon used for adsorption of organic impurities from waste water.
2. Brief Description of the Prior Art
A commonly experienced problem is the contamination of granular solids with impurities, particularly organic impurities. The problem is encountered when soils become contaminated by spills of industrial chemicals and when waste industrial chemicals have been indiscriminately been discharged to waste sites. An efficient method is required for regeneration of the contaminated soil.
The problem also occurs in the contemporary industrial practice of treatment of waste water with solid adsorbents--predominately activated carbon--to adsorb organic impurities from the water, prior to discharging the water to the environment.
The adsorption of organic compounds on granular solids, such as the clay and loam of soils, and on other industrial adsorbents such as clays, silicas, activated carbon, etc., is a physical phenomenon; organic compounds are attracted by Van Der Waals forces and become physically attached to the surfaces of the granular solids.
The solid adsorbents which are used to purify waste water become spent when organic compounds are adsorbed on the surfaces of the solids, and the spent adsorbents must be discarded or regenerated. Regeneration of activated carbon is practiced only in limited applications, and the majority of spent activated carbon is trucked to secluded waste disposal sites. The same disposal is practiced with soils which have become contaminated with organic impurities.
The most commonly used regeneration for activated carbon has been thermal desorption in which the activated carbon is placed in a retort and heated to elevated temperatures, typically about 1000 degrees C. While this treatment effectively regenerates the activated carbon, it is quite costly in initial investment and operation. Additionally, the operation of the waste water treatment plant must be interrupted to remove the spent adsorbent from the adsorption vessel to transport the adsorbent to a remote location for the thermal desorption. Because of the high capital investment and operating costs experienced in thermal regeneration of activated carbon, most of the activated carbon used to treat waste water is discarded in hazardous disposal sites.
Disposal of contaminated soil, or spent adsorbents such as activated carbon at hazardous disposal sites is not, however, environmentally acceptable, particularly in view of the decreasing availability of suitable land sites for disposal, and public sentiments.
Other techniques for regeneration of activated carbon which have been attempted include vacuum and solvent purging wherein the spent adsorbent is subjected to reduced pressures and/or contact with organic solvents. Another technique which has been used is adsorbent displacement by contacting the spent adsorbent with another adsorbent.
Of the aforementioned treatments, the thermal regeneration is most widely practiced, however, the vast majority of spent activated carbon is disposed without regeneration.
There have been suggestions in the prior art that ozone can be used for gas phase oxidization regeneration of activated carbon, e.g., see Japanese Patent Application 76-140713 and German Democratic Republic Patent DD237,116. Ozone has also been used for regeneration of cuprous sulfate impregnated activated carbon as disclosed in Japanese Application 74-379185 and 78-114904.
Ultrasonic vibration has been applied for the regeneration of chromous-cupric adsorbent; see Przem, Chem. 62(9), p 518 (1983). Ultrasonic treatment has also been suggested as an aid in ion exchange regeneration in French Patent 2093541.
Heretofore, there has been no technique practiced for the regeneration of spent activated carbon adsorbent that would economically remove organic adsorbate in an efficient manner which could eliminate all potential environmental objections.